Curving flexure-armature



BOUNCE DISTANCE("PUNCH/"NO-PUNCH")IN u s July 1, 1969 s so 3,453,572

CURVING FLEXURE-ARMATURE Filed Dec. 5, 1967 Sheet of 2 FIG. 4

INVENTOR.

' I 8 2? EARL E. MASTERSC SPACER THlCKNESS (MILS) BY I ATTORNEY 4 July1, 1969 E. E. MASTERSON 3,453,572

CURVING FLEXURE-ARMATURE Filed Dec. '5, 1967 Sheet L of 2 INVENTOR. l 5EARL E. MASTERSON ATTORNEY United States Patent US. Cl. 335276 8 ClaimsABSTRACT OF THE DISCLOSURE A solenoid having an elongate flexure-leafarmature to be selectively attracted, pivotingly, against a pole-facesurface wherein a relative curvature is introduced between flexure andpole surface so as to create a curvingly-divergent gap.

Problems, invention features This application is a continuation-in-part:of my US. patent application Ser. No. 565,309 filed July 14, 1966which, in turn, was a continuation-in-part of my US. patent applicationSer. No. 402,412 filed Oct. 8, 1964, issued in October 1966, as US.3,279,690; the disclosures of both being hereby incorporated byreference in this case.

Workers in the art of providing solenoid magnetic actuators, such asthose described in the aforementioned application, are well aware that acommon shortcoming in the operation of such actuators is the manner oftypical impact of the armature with seat or stop members. For instance,it is common knowledge that such impact can result in an armature bouncecondition, introducing all sorts of undesirable jitter and otheruncertainties in armature action. It is also known that such impactusually wears these impacting members to a significant degree,especially where one of the impact surfaces comprises a (typically soft)ferrous magnet-core. Thus, it is particu- 'larly desirable in this artto provide solenoid actuators minimizing undesirable armature-impact.The present invention is directed to improvement features effectuatingthis.

More particularly, according to the present invention, I have found thatpivoting a flexible armature strip so as to establish a prescribeddivergent working gap with respect to a magnet pole-surface andfashioning one or both of the strip and the surface to make thisdivergence non-linear (such as by curving the pole-surface), achieves adesirable mode of armature/pole impact. Specifically with this structurethe armature is gradually rolled down the pole surface (or wrapped)rather than being slapped against it, etc. as in the conventional case.

Besides the evident advantages attendant on such a wrappingarmature-engagement, it should also be understood that in many cases,the wrapping, resilient armatu-re provides a more eflicient operationmagnetically; that is, because the armature strip is flexible andbecause the strip and pole-surface typically curve away from oneanother, the working flux passing between them, may, in certain cases,be thought of as pulling small incremental segments of the stripsomewhat independently, with the non-linear, divergent relation helpingto concentrate the flux closer to the area of engagement and with theresiliency of the strip helping to lighten the attracted load somewhat.

Objects, features Thus, it is a general object of the present inventionto provide a device which exhibits the aforementioned features andadvantages and which alleviates the aforementioned problems andobjectionable characteristics. An object is to provide a device whichoperates to attract an elongate flexure-strip armature somewhatgradually into contact with a magnetic pole-face, to engage positivelyand yet without noisy or damaging impact. A related object is toincrease solenoid efliciency by concentrating flux along shortsuccessive segments of a length of flexiblc armature strip.

Another object is to provide such a device having an armature whichwraps gradually onto a pole-face. Still another object is to providearmature and pole-face which are fabricated to curve away from oneanother in a nonlinear divergent gap. Still another object is to providesuch a curved pole-face portion by provision of a bent flexure strip.Still a further object is to provide such a device wherein the armaturestrip and an associated guide strip are mounted in common with aprescribed spacer therebetween. Still a further object to provide such adevice having the strips of dilferential length and the thickness of thespacer therebetween adjusted accordingly so as to accommodate thiswrapping engagement mode, at least adjacent the free-end of thearmature. Other objects will become more apparent as the followingdisclosure proceeds.

The foregoing and related objects, features and advantages of theinvention will become more apparent from consideration of the followingdetailed specification including the accompanying drawings, indicating apreferred manner of making and using the invention; and wherein one typeof embodiment may be described in a somewhat simplified manner asgenerally comprising: a flexure armature/solenoid combination whereinthe engaging portion of the magnetic pole-face is curved so that thearmature strip may be gradually attracted thereby to be curvilinearlyengaged, or wrapped, onto a preferred contact-plane operatively adjacentthe pole-face and also, preferably, includes a flexure-guide, orpre-form, overlaid on the pole-face to define this curved contact plane.

The foregoing and other novel features, advantages and characteristicsof the invention together with those related will become more apparentto those skilled in the art from the following disclosure.

The drawings, wherein like reference characters denote like parts,comprise:

FIGURE 1, an upper perspective view of one embodiment of the invention,somewhat simplified for clarity;

FIGURE 2, a schematic side section of an embodiment like that in FIGURE1 but somewhat modified to include a flexure guide;

FIGURE 3, a simplified, fragmentary side elevation of an embodiment likethat in FIGURE 2, further modified to include a pair of guides andintermediate spacer;

FIGURE 4, a plot of bounce distance as a function of spacer thickness inthe typical operation of solenoids like that in FIGURE 3;

FIGURE 5, a very schematic side-view, in partial se tion, of anembodiment similar to that in FIGURE 1 but modified to function in aratchet and pawl embodiment; and

FIGURE 6, a fragmentary, upper perspective of a set of solenoids, eachafter the manner of FIGURE 1, but modified somewhat to comprise amutliple print-hammer actuator embodiment; FIGURE 6A indicating amodified version of one of the solenoids thereof.

In general, embodiment A-1 in FIGURE 1 comprises a solenoid-actuatorassembly including a flexure-armature f and a rather U-shaped magnetstructure, or core M, around one pole (leg) of which a prescribed coilal is wrapped. Ooil al will be understood as being conventionallycurrent-pulsed (leads 11) so as to project an electromagnetic flux forattracting armature 1 toward pole face surface pf, this attraction beingquite conventionally eifected by intersecting flux from pf according tophenomena well known in the art. The other leg of magnet M (or anyconvenient fixed 'base) is arranged to fixedly mount armature-fiexurestrip (such as with the indicated clamp C on pole-face pf of the othercore leg) so as to be cantilevered out in flux-intercepting(attractable) relation with this attracting magnetic flux projected frompole-face pf. Flexure armature will thus be understood as normallyassuming a diverging gap relation with face pf so that its free tip ftis spaced the farthest therefrom (f being in rest, or unattracted,condition, in the phantom representation of FIGURE 1). Strip isfabricated to comprise an elongate fiat strip (flat-leaf fiexure)armature of flexible, magnetizable material, such as magnetic springsteel. Workers in the art will understand that the operation of such afiexure-armature solenoid A-l will be intended to be such that when aprescribed attract-current pulse ip be applied to coil cl (atactuate-time T sufficient fiux will then be projected from pole-face pfso as to attract the intercepting portion of strip ft toward face pf(and into contact therewith in the ordinary case, as illustrated inFIGURE 1 in fullline). The flux-traversing gap gt between armature andpole-face is characterized as the working gap.

Curvilinear-di verging gap It will be apparent, however, that onefeature of the construcution of A-l is that pole-face pf and fiexure fare arranged so that pf presents a relatively curved contact plane withrespect to the normally-flat plane of the (unattracted) strip 1'' (asmore particularly described below, such as with respect to theembodiments in FIGURES 2 and 3). Thus, upon the energization of coil cland the consequent generation of flux from face pf, strip f will beginto wrap itself down onto face p7, progressively contacting it along itslength. Such a gradual, incremental armature-pole-engagement may becompared with the familiar slapping of prior art armature structures,even those with (linearly) diverging working gaps, wherein afiat-contact plane is established, along which some or all of thearmature-length may be simultaneously (rather than progressively)impacted. This curvilinear-diverging gap feature will thus effect agradual, incremental (bending) attraction of fiexure f, progressivelypulling-in, closer segments thereof with increased, localized force(inverse-square variation of flux attraction with gap size) so thatthese segments will be rolled-down over p7 (with the pivot-points movingdown along p7 accordingly). It will be understood that fiexure strip 1will thus be made to assumea prescribed flexing mode so as to have aprescribed known bending radius along the locus of the contact-plane andthat pole-face p f will be accordingly fashioned to conform to it.

FIGURE 2 indicates a modified fiexure-armature solenoid (actuator)embodiment A4, to be understood as generally the same in structure andoperation as A-l in FIGURE 1 except where noted. Thus, solenoid A-2includes a magnetizable fiexure-strip armature f-a afiixed to a fixedbase member 6-11 to be cantilevered out therefrom in prescribed,magnetically-attractable relation with the pole-face pf-w of associated,solenoid-energized, magnet pole MP of a magnet core (only pole MP beingshown). Pole-face pf-a is curved in the manner of pf (FIGURE 1) so thatupon (selective) generation of magnetic fiux in MP it may project fluxfrom pf-a sufficient to attract armature f-a so as to wrap itselfprogressively onto pf-a in the aforedescribed manner. Strip f-a will bedisposed to (in unattracted condition, the full-line showing FIGURE 2)establish a prescribed curvilinear working-gap separation from facepf-a, this gap increasing non-linearly (exponentially) along the lengthof f-a to assume a prescribed maximum-gap dimension Z-gt at the topthereof. However, according to a modification feature, strip fa is notarranged to be normally flat (as in the case at s rip pf in FIGURE t ispmen as indicated (in full) away from p'fa', to have a prescribed radiusand thereby accenutate the non-linearity of gap divergence. Thisconstruction will be convenient where it is not convenient to curve facepf-w very muchindeed where strip f-a is so pre-bent, pole-face pf-a mayitself be flat and still effect the wrapping contact desired.

In some cases, it may not be convenient to fabricate face pf-a to aprecise radius (e.g. machining expense), and, for instance, be desirableto fashion face p f-a to conform only roughly to the prescribedcurvature. In some cases, it will also be desirable to prevent fiexuref-a from contacting face pf-a however gently (e.g. risk of some abrasionetc. of typically-soft core; desirability of quick, nonsticky release off-a, etc.). In either such case, one may provide a pole-covering guidemeans such as non-magnetic fiexure guide f-g according to anotherfeature of the invention.

For such cases, it will be preferable, as indicated here, to overlaypole-face pfa with guide strip f-g clamped between a first base-clampc-a (with f-a) and a second base clamp cb* such as to assume aprescribed (pre-bent) curved profile, and contact-plane, along whicharmature strip f-a may wrap. The flux generated from pole MP will thuspull armature strip fa gradually and wrappmgly down onto guide f-g (thebulk of the flux passlng through f-g). Ordinarily, one would assume thatat least a portion of the gap between pole-face pf-w and magnetizablestrip f-a should be open (i.e. left unintercepted by the body of guidef-g) for optimum magnetic flux-flow and inter-attraction. However, ithas been found that this is not always necessary. For instance, goodmagnetic action has been experienced using a guide, analogous to f-g,comprised of non-magnetic material about 18-20 mils thick and preferablyhaving high-flatness (e.g. about .003 TIR on the armature-engagingsurface-see also aforementiond applications S.N. 402,412 and 565,309).

In'embodiment A-1 above, an exponentially diverging (armature/ pole) gapwas rendered according to the curvature of the pole relative to thecomparatively flat armature-strip. Of course, it will be understood thatthis strip need not be flat in all cases, but may, itself, be curved,such as in the case of strip fa in FIGURE 2. Of course, where thearmature is somewhat flatter than pole-face pf (that is, had a greaterradius of curvature), it is more apt to wrap itself gradually intocontact therewith. Contrary-wise, embodiment A-2 indicates how thisnon-linear gap-divergence may be accentuated by pre-bending the fiexure(e.g. f-a) to curve away from the contact plane (with the magnetic polemeans MP) to have an opposite radius of curvature. The resultant gap(see 2-gt at the armature-tip) is somewhat exaggerated here forillustration purposes. Itwill be understood that such a pre-bending offiexure strip f-a may be effected in the manufacture thereof or in themounting thereof, such as with a clamp (not shown) adapted to cause therequired bend.

A related solenoid embodiment A-3 is shown in FIG- URE 3 and will beunderstood as constructed and operating like the foregoing embodimentsexcept where noted. While not necessarily including the non-lineardiverging gap feature, A-3 in general exhibits a pre-bent fiexure striparmature f-b together with a bending fiexure guide (or 'pre-form PRb-afeature more particularly described and explained in co-pending US.application Ser. No. 565,309) and, according to another feature of theinvention, exhibiting a curved fiexure/pole gap section. Embodiment A3will be recognized as comprising an attracting magnetic pole memberMP-b, a non-magnetic curved guide (or pole-contact member, i.e. contactpreform PR-b), cantilevered-out from a fixed base C-b. Clamped to thesame base (e.g. by clamp C-b', schematically indicated only) is magneticfiexure strip armature f-b, separated by a prescribed spacer Sp fromguide PR-ln A o c p d in this man er, and. sup p ed bendingly againststrip f-b, is the bending-preform PR-b (clamped in common by clamp C-b',cooperating as known in the art, with fixed base C-b). Guide PR-b willbe understood as constrained (by schematically indicated clamp F-i) soas to assume a prescribed bent curvature. Similarly, bending pre-formPR-b will be understood as pre-bent to assume a prescribed curvaturesimilar to that of guide PR-b being understood as bent by anyconventional means (e.g. stop-surface indicated functionally at arrowF-ii in phantom). Bending pre-form PR'-b is, of course, arranged topre-bend fiexure f-b in its static (unactuated) state. Flexure-armaturefb will thus be pre-bent to have a radius similar to that of inner guidePRb, being somewhat shorter and cantilevered from the same clampingpoint C-b. Where the flexure armature is the same length as the contactpre-form PR-b, a spacer SP will not be necessary; where they differ,however (e.g. FIGURES 2, 3), I have found, according to another featurethat it is highly desirable to introduce a spacer SP of prescribedseparating-thickness to improve their mode of engagement.

Where spacer SP is used, I have found that its thickness is relativelycritical and depends upon the length differential. For instance, in anarrangement like A-3, if spacer-thickness gs is too small (or SP isomitted entirely), problems will arise in operation of flexure f-b; forinstance, its contact with PRb will be too intimate over too great asurface area for quick-release and can also present an undesirably largebending moment (depending on the stiffness of f-b), e.g. such thateither too much magnetic flux is required to attract it or else suchthat f-b will not wrap completely along PR-6 and, typically, neverengage at its tip, ft.

Conversely, if spacer SP is overly thick, and/or if strip f-b is notstiff enough relative the applied magnetic force, the strip may tend tocontact PR-b unevenly and not gradually, tending to slap its tip ftagainst PR-b prematurely (rather than gradually wrapping along itslength). It will be apparent that in such a case, damagingcontact-abrasion, noise bounce (cf. below) and the like will beencouraged and that too little surface area of strip f-b will engagewith the guide, tending to allow disengagement prematurely (e.g. bounce;poor hold forces, etc.). Thus, a spacer SP having a prescribed thicknessrange (e.g. as suggested below) is found preferable according to theinvention; for instance, such a thickness as will cause the cantileveredlength of strip f-b to contact guide PR-b gradually and continuously,along most of this length (i.e. substantially to its outer free end).

Moreover, in cases of a length-differential (different lengths ofarmature strip f-b and pre-form PRb as aforementioned) I have found,according to this feature, a particular need for spacer SP to effectbetter engagement of these members. That is, Where pre-form PR-b issignificantly longer than strip ,fb (assuming that bending guide PR'bkeeps them relatively parallel), the strip will tend to buckle alongPR-b, leaving an intermediate portion of its length standing off fromPR-b, humplike, when its tip ft has engaged PR-b (this engagement isalso more apt to be a slapping, nongradual, one, inducing bounceas seenbelow). The greater this differential is, the thicker SP must be. On theother hand, where guide preform PR-b is significantly shorter than stripf-b, it will tend to slap its tip it prematurely against PR4) (in theaforementioned manner, with the described effects) and not wrapgradually therealong.

The foregoing limitations on spacer thickness may be illustrated byreference to particular dimensional embodiments for arrangement A-3. Forinstance, spacer thickness may be adjusted, according to a particularfeature of the invention, to minimize armature bounced This may beunderstood with reference to the curve in FIGURE 4, where I have foundthe following: Using U- shaped clamp C-b' etc. and an armature strip f-blike that in the aforementiond applications (about 1% in. eff. lengthand about A% in. wide and 12 mils thick, with guide PR-b about A1 in.longer) together with a spacer SP about 20 mils thick, a serious bouncecondition was discovered whereby strip f-b would be attracted bysolenoid pole MP-b, when the latter was energized, so that its tipslapped against guide PR-b such as to rebound one or more times (notethe danger of accidentally rebounding back towards PRb, i.e. into the oncondition, and thereby inducing accidental, erroneous actuations, etc.).The amplitude of such bounces were severe enough to comprise about 10mils or more where the nominal minimum armature/guide gap gt wasarranged to 'be only about 18 to 25 mils-introducing the risk ofaccidental actuation. This condition is reflected in curve 4I where the20 mil spacer could produce a bounce distance in the neighborhood ofabout 7 mils. It was also found that reducing the spacer thickness toomuch also produced an undesirable amount of bounce (of. curve 4-D andthat there was an intermediate thickness range 4-R from about 10 to 18mils (prefer 10-15) that was optimum for minimizing bounce in thisembodiment. This corroborates the aforementioned thickness critically,although the underlying phenomena are not wholly understood as yet.

Of course, bounce may also be reduced more conventionally, such as byincreasing the attracting magnetic force, decreasing the effective gapbetween armautre f-b and magnet MP-b etc. It was also found thatreducing the thickness of strip f-b from about 12 mils to about 10 milsreduced the amount of bounce (presumably because this makes the air gapabout 2 mils less and the magnet pole MP-b more effective in movingstrip f-b which is thus rendered less stiff). Of course, asaforementioned, one must be careful not to force too great an area ofstrip f-b into intimate engagement with preform PR-b since the releasetime and associated energy will then be too great, and release tooslow). One way of avoiding this is to increase spacer thickness, asidefrom other considerations (such as length differential).

Another embodiment of the invention is indicated in FIGURE 5 where amodified solenoid actuator A-S is shown, being intended to function inthe manner of a pawl actuator for a conventional (schematized) ratchetwheel RW. Here actuator A-5 takes the form of a fiexure armaturesolenoid with the tip of the armature flexure strip S-A being fittedwith a detent PA arranged to be indexed registeringly into notches i cutin a periphery of ratchet wheel RW. The general operation of thisarrangement will be well known in the art whereby energization ofactuator A-S will drive dentent PA (pawl-like) into and out of notches ito control the rotation of wheel RW (arrow). Actuator A-5 is like theaforedescribed solenoid actuators except as noted and generallycomprises a solenoid magnet S-M mounted on a fixed plate 5B (in anyconvenient manner, so as to assume a fixed position for control of RW),magnet 5-M inculding a prescribed pole-leg adapted to present a (veryslightly indicated) curved pole surface 5-P in operation relation with aportion of strip 5-A and to be selectively energized by associatedsolenoid coil 5-C as known in the art. Flexure strip armature 5-A ismagnetic and resilient, being clamped onto magnet 5-M to be cantileveredout in confronting relation with curved surface 5-P and also,preferably, pre-bent so as to create a non-linear diverging gaptherebetween in the aforedescribed manner and to be wrapped graduallyand continously thereonto when attracted by emanating magnetic flux.This gap is established acocrding to the relative curvature of pole face5-P (here indicated as being relatively flat, though it could have beencurved) and by the clearance of PA with RW in rest condition. It will beseen as very useful to arrange strip 5-A so as to present a stiff-armresistance to the rotation of wheel RW when detent PA is locked in anassociated notch i (i.e. not buckle). It will be assumed that in itsnormal, static, condition, strip 5-A will present detent PA so that itwall fall into a registering notch i and oppose the rotation of wheelRW. Thus, upon the energization of coil -C, strip 5A will be attractedto be wrapped, somewhat gradually and continuously, along curvedpole-face S-P and thereby remove detent PA from stopping engagement withwheel RW (e.g. RW may be continuously urged for this, such as by aspring-drive). The current through 5C would be applied until wheel RW isto be stopped whereupon its cessation would release armature S-A tospring back toward wheel RW, and allow detent PA to seat in a followingassociated notch 1' Armature 5A should, preferably, be of flexible,durable metal, such as of hardened steel, and will thus have less thanperfect magnetic properties, however, still being operable. It may behelpful to taper the cross section of 5-A to make it flex in a truer arc(e.g. per FIGURE 6) and thus produced constant stress along its lengthas it conforms to the desired arcuate locus. As known in the art, ananti-stick shim surface may 'be provided on at least a portion ofarmature 5-A (or any like armature) for quick-releasing engagement withpole 5-1. Also, a nonmagnetic spacer ns may be provided over the cavityreceiving coil 5C, offering a guide surface for strip 5-A to wrap uponso that it doesnt buckle there.

Another modified embodiment of the invention is indicated in FIGURE 6where a plurality of solenoid actuators, somewhat like thoseaforedescribed, are shown as formed into an integral actuator set A-10functioning in the manner of a set of print-hammer solenoid actuatorsfor use in a relatively conventional high-speed printer arrangement.This arrangement will be understood as including a conventional drum PR(schematically indicated) with each actuator 10A in the set A-10 takingthe form of a flexure-armature solenoid with the tip of thearmature-flexurestrip 10-S being adapted to present a print-hammer H atthe free tip thereof. In general, print drum PR may be understood asconventionally presenting successive rows of raised-type font for theselective impacting thereof by an associated hammer H at a prescribedselect time. As understood in the art, when particular printing media(e.g. paper and ribbon) is presented between the set of hammers H anddrum PR (such media not shown, but well understood in the art), propertimed energization of coils cl associated with prescribed actuators 10-Awill effect printing on this media at an associated print-columnposition thereon (by the impacting of hammer H onto the paper andagainst the type-font, then positioned behind it, etc.). Except wherespecified, each actuator 10-A will be understood as functioning in themanner of those aforedescribed, and including a magnet portion 10M, acoil cl and a magnetizable flexure-strip armature 10A pivotingly mountedon the working pole-face of the magnet so that with a non-lineardiverging gap established between this strip and the poleface asindicated in the drawing by the curvature of the pole-face, a gradualcontinual wrapping engagement of this strip onto the pole-face will beeffected to, in turn, impact the hammer portion H for printing. Strips10-A (e.g. 10-A1 with magnet 10-M1 and hammer H-1) may be mounted to soproject above the associated pole-face in any convenient manner such asby clamping attachment to one end of the pole-face as indicated and willbe arranged to project beyond the pole-face portion to effect thisimpact with the print-roll PR described. The integral set of actuators10-A may be mounted in alignment such as by provision of an alignmentbore 10-0 through which a common mounting means may be registeringlyinserted or the like.

For optimal magnetic isolation between actuators, nonmagnetic spacerblocks 10-SP (e.g. 10SP1 between solenoids 10-S1 and 10-S2), may beprovided therebetween as indicated. This, of course, means that eachactuator magnet block 10-M will be quite narrow having a width on theorder of the width of a typical print-column (somewhat less where blocks10-SP are to be accommodated).

It will be recognized that the simple uncluttered configuration shownwill lend itself to the provision of such miniaturized actuators incompact environments like the printer indicated. It will be recognizedthat among other advantages the provision of the described non-linear,diverging working gap will desirably provide a gradual controlledimpacting of the hammer head, together with a controlled self-biasedreturn thereof.

Where advantageous, each individual actuator 10S may be fabricated asindicated in FIGURE 6A according to a modified simplified structure tocomprise a central, permanent-magnet core m (such as a ceramic magnetpoled to attract the associated flexure strip H in a prescribeddirection, toward, or away from, the assembly) and a pair ofmagnetizable, low reluctance pole plates P, P, attached on either sideof magnet M to effective- 1y form coil-receiving, flux-directingmembers, between which solenoid coil 0 may be wrapped for magneticenergization of the assembly.

Having now described the invention, what is claimed as new and intendedto be secured by Letters Patent is:

1. An electromagnetic actuator assembly for use in connection with aworking surface, said assembly comprisa magnetic core having anoutwardly curved pole face;

flux means for selectively generating magnetic flux within said core;

means forming a working element in selected disposition relative to saidworking surface;

a flexible elongate magnetic armature having a first end mounted infixed relationship to said core and a second end fixed to said workingelement, said armature being suspended over said pole face to define anon-linearly diverging working gap therebetween;

whereby generation of a predetermined magnetic flux level within saidcore attracts said armature into gradual, progressive contact along thecurved pole face to move said working element into low rebound impactwith said working surface.

2. An electromagnetic actuator assembly as described in claim 1 wherein:

the curvature of said pole face and the shape of said armature define anexponentially diverging working gap.

3. An electromagnetic actuator assembly as described in claim 1 wherein:

said flexible armature comprises a narrow strip, curved between saidfirst end and said second end.

4. An electromagnetic actuator assembly as described in claim 1 wherein:

said core is supported within an outer frame upon which the first end ofsaid armature is mounted; and

said curved pole face is masked by a thin correspondingly curved stripfixed to said frame.

5. An electromagnetic actuator assembly as described in claim 4 whereinsaid strip is formed of non-magnetic material.

6. An electromagnetic actuator assembly as described in claim 4 wherein:

a spacer is fixed between the first end of said armature and said frameto provide a working gap of predetermined minimum dimension; and

a preformed elongate guide is fixed to said frame, overlying thearmature, whereby said armature will conform to the shape of said guidewhen the flux means are unenergized.

7. An electromagnetic actuator assembly as described in claim 1 wherein:

said working element comprises a print hammer adapted to move into lowrebound contact with a print drum when said actuator is energized.

8. An electromagnetic actuator assembly as described in claim 1 wherein:

9 r 10 said working element comprises a detent adapted to be FOREIGNPATENTS Withdrawn from one of a plurality of notches in the 436 71710/1935 Great Britain periphery of a rotatable member when said actuator688:832 6/1940 Germany is energized.

References Cited 5 G. HARRIS, Primary Examiner.

UNITED STATES PATENTS U.S. c1. X.R. 3,172,022 3/1965 Tibbetts 335-235 XR335-279

